The present invention relates to a sheet material for ink-jet printing or, more particularly, to a sheet material for ink-jet printing having greatly improved printability by virtue of a surface coating.
As is well know, many types of hard-copying apparatuses for letters including Chinese characters, graphic images and the like as well as facsimile machines, word processors, computer terminals and the like utilize an ink-jet printer as an output recording or printing machine. Ink-jet printing is a method in which a printing ink of low viscosity is pressurized and ejected from a plurality of nozzles each of a small diameter of a few tens of micrometers in the form of fine droplets having a diameter of, usually a few tens of micrometers or, in some cases, from 100 to 200 micrometers depending on the pressure, diameter of the nozzles, electric power for electrostatic charging of the droplets and so on and the desired images are formed by the impingement and pattern-wise deposition of the ink droplets, which are under control relative to occurrence and direction and velocity of flying out of the nozzles, in dots on a sheet material for ink-jet printing. Ink-jet printers are more and more widely used in recent years by virtue of the economical advantages over conventional printing methods with a quality of the printed materialst equivalent to other printing methods.
One of the problems in the ink-jet printing method is the printability of the sheet material for printing on which the ink droplets are deposited in dots. For example, the ink-jet printing is performed by using an aqueous ink so that the printing sheet having received the ink droplets deposited thereon must absorb water in the ink droplets as quickly as possible in order that the ink can rapidly permeate into the sheet. This requirement is important in order to prevent troubles in a printed matter on a poorly water-absorptive printing sheet by smearing when it is contacted with, e.g., fingers, before long after printing and stain of the contacting body as well as to prevent overspreading of the dots leading to a blurred pattern with a decrease in the color density. Accordingly, it is a conventional practice when the base material of the printing sheet having poor water-absorptivity such as a plastic film is to be used as the base sheet for a printing sheet of ink-jet printing that the surface of the sheet is coated with a coating liquid which is a dispersion of a fine powder of a highly water-absorptive resin or an inorganic water-absorptive filler in an organic resinous material as a binder or vehicle to exhibit adhesion to the surface of the base sheet. When the content of the organic binder in the coating liquid is too high relative to the water-absorptive powder, the water-absorptivity of the coating layer is poor to exhibit repellency against the ink droplets falling thereon resulting in blur of the dots or an unduly small diameter of the dots with consequently decreased effective density of the patterned images. When the content of the organic binder is too low relative to the water-absorptive powder, on the other hand, the particles of the water-absorptive powder are poorly dispersed in the binder as the vehicle sometimes to cause agglomeration so that the ink dots may have a somewhat increased diameter to decrease the resolving power of the image if not to mention the poor adhesion between the surface of the base sheet and the coating layer which is accordingly subject to falling by rubbing.